Rear Disc Brakes and the Prop Spring

I was told that to keep my rear discs (off a Grand Cherokee) from dragging all the time, that I needed to remove the spring from inside the proportioning valve on the rear brake side. I tried the brakes first without fooling with the prop valve and they seemed to work well. The next day... thinking they might work even better, I pulled the spring. With the rear off the ground, and the tranny in D, I couldn't keep the tires from spinning... even with my foot to the floor. (And I bled the brakes). Frustrated... I threw the spring back in (easier to take out) and tested it without even bleeding. It stopped really well again. I'm sure with bleeding even better.

So the moral? If you have a 93 XJ, and you put ZJ discs in... don't take out the prop valve spring.

i agree
i just did GC disc conversion on my '99 w/ a '89 d-44 swap & i'm very pleased w/ its stopping power for 35s w/ the proportioning valve un-touched

I do believe you are supposed to remove the o ring on that side, but keep the spring there. I haven't touched my prop valve and im very happy with the setup.

I removed that o ring and the rear locks up a lot better now. I tested it before and after in the dirt and there is a noticeable difference.

So removing the o-ring makes it even better? Where is the o-ring? When I pull of the cover I see a spring and a brass looking cylinder inside. The spring pulls out. The cylinder will wiggle but I have not tried to pull it out. Is the o-ring in there?

Harvo said:

So removing the o-ring makes it even better? Where is the o-ring? When I pull of the cover I see a spring and a brass looking cylinder inside. The spring pulls out. The cylinder will wiggle but I have not tried to pull it out. Is the o-ring in there?

Click to expand...

Yeah watch out that spring does'nt get away from you. Grab that rod and slide it out, on the end is a black o ring. Pull it off and slide the rod and spring back in. Then bleed it well!!!! This will allow the rear to lock up easier.

Harvo said:

So removing the o-ring makes it even better? Where is the o-ring? When I pull of the cover I see a spring and a brass looking cylinder inside. The spring pulls out. The cylinder will wiggle but I have not tried to pull it out. Is the o-ring in there?

Click to expand...

I did the ZJ discs too.
I replaced the valve with a ZJ valve, everything works fine. By doing that, the system works as originally designed.
By keeping the original prop valve as is or modifying it by removing the seal, you are redesigning the whole thing.
I don't know enough to do that, so I kept it to factory specs.

So you also changed to a grand master cylinder?

I went to corvett master and hydroboost, works great, still does not lock the rear before the front but might mess with prop valve or replace.

Bigwhitey- Looks like your prop valve is mounted 90 degrees from mine on it's side. What year is your XJ?

XJJack said:

So you also changed to a grand master cylinder?

I went to corvett master and hydroboost, works great, still does not lock the rear before the front but might mess with prop valve or replace.

Click to expand...

The XJ I converted is a '96, AFAIK the master cylinder and booster are the same as the ZJ. Once I replaced the rear drums, the only item left that was not a ZJ was the valve. I replaced the valve with the ZJ part and now I believe that the brake system is all ZJ. Some secondary parts had to be modified, but all the primary are ZJ.

Harvo said:

Bigwhitey- Looks like your prop valve is mounted 90 degrees from mine on it's side. What year is your XJ?

Click to expand...

Thats not mine it's an example of how to get the rod out.

I've done the ZJ disc conversion on my '90 as well. big difference after I removed the o-ring. I can lock up all 4 of my 31" mt/r's with the backs allways locking first. and this is with the single diaphram booster. should be an effortless dream after the double diaphram swap!

Just pulled all the ZJ parts from the boneyard, minus the rotors for get this.....$36 bucks!. I am going to convert my cherokee 44 under a 2000 XJ 4.0 and I think at this point i need to know which parts i lack:

I have:
calipers including short rubber hose to connect to hard line and the mounting tab that held the hose to the spring perch on the ZJ
caliper bracket
backing plate
ebrake "lever" that I hear doesn't come with the rebuild kits

I need:
ebrake rebuild kits
new ebrake shoes
rotors
zj proportioning valve
rubicon D44 retainer plates


To do:
ream out the hole in the backing plate
re-drill the mounting holes to fit the d44 flange
mod my existing YJ cables to work with the ZJ lever
replace proportioning valve (how?)
get rotors from NAPA, etc.
get ebrake rebuild kits and shoes
press off/on bearings and new retainer plate

Good news is i have a spare D44 and master cylinder to mock everything up.

What am I missing? Any gotcha's out there? Advice on redrilling the caliper bracket?

-P

xxxj-va said:

replace proportioning valve (how?)

Click to expand...

When you get the ZJ valve it will most likely come with the bracket that attaches the whole assembly to the booster. That bracket has a rectangular hole, the aluminum valve sticks in that hole. The portion of the valve that sticks out of the bracket has been swaged to prevent the valve from dropping out.
Remove the swaged aluminum by filing or chiseling (no big deal) do the same thing with your original valve and mount the ZJ valve in the XJ bracket.
Use a center punch to stake the ZJ valve in the bracket.

I see exactly what you mean.....much appreciated.